1. Field of the Invention
The present invention relates to a method for removing ammonia or reducing ammonium, respectively, from fermentation liquids or fermentation residues used in the production of biogas.
2. Background of the Invention
The energy-related utilization of waste products and by-products has been gaining more importance in the sustainable production of consumer goods. In particular in the field of food production, any occurring organic waste may be used, by making use of the biogas technology, in order to (partially) cover the energy requirements of the production. In this way, the digestion of waste in the framework of production-integrated environmental protection and the production of renewable energy may contribute to the sustainable development of the company. On a national level, such measures are also desirable, as they contribute to the “2020 climate objectives” stipulated by the European Union.
The biogas that is formed by the residual substances accumulating in the production may be converted into electrical current and heat in a combined heat and power plant (BHKW). Alternatively, biogas could be processed, directly substituting fossil natural gas. By integration in an operation it may be guaranteed that the thermal energy is used all over the year, in this way guaranteeing the efficient use of the energy carrier with high overall efficiency. As a positive side effect, the energy-consuming disposal of these materials may be omitted due their high water content, and valuable nutrients like nitrogen, phosphorus and potassium may be returned to agricultural areas, together with the fermentation residues.
The internationally increasing demand for meat and meat products and the production associated therewith is responsible for the increased production of non-edible animal by-products. Offal as possible new substrates of industrial origin show a high potential for the production of methane and are in no way in competition with the food production sector. These and many more similar residual substances such as, e.g., waste from the pharmaceutical industry as well as blood processing (plasma waste), waste from the protein and meat processing industry and yeasts and slurries from the fermentation of ethanol, however, may be processed using prior art technology only under difficult conditions in anaerobic waste treatment plants.
In many regions the soils are supplied with high amounts of nitrogen due to intensive agriculture and animal breeding. Additional nutrient introduction may entail increased eutrophication of the waters and pollution of the groundwater. Methods for removing nitrogen from biogas fermentation residues would enable for the concentration of the developing ammonia into a transportable form, which may be transported off the region and thus reduce the impact on nature. In this way, the biogas production could be increasingly used in regions, where it is difficult to use due to lacking agricultural areas.
A major problem on the way to an overall implementation of the anaerobic technology in the energy-related digestion of, e.g., offal is the high nitrogen content thereof. The degradation products of the nitrogen compounds ammonia (NH3) and ammonium (NH4+) that are contained in the substrate will lead to inhibition of the microbiology when exceeding certain concentrations. These toxic substances lead to a reduced degradation of the used waste substances and, as a consequence, to an unstable biogas process. This is why the nitrogen containing substrates were frequently transferred to external biogas plants for co-fermentation, whereby the possibility of integrating the waste heat of the combined heat and power plant in the production operation is omitted and the total efficiency of the slaughter house is reduced.
The process described in this patent document constitutes a key technology for the anaerobic digestion of nitrogen-rich waste and by-products. The removal of the nitrogen charge from the biogas process guarantees a stable and efficient process procedure.
In the context of renewable energy and environmental protection on the company level, biogas plays an important role. One example thereof is the slaughtering and meat processing industry. Non-edible animal by-products offer huge energetic potential in the anaerobic digestion. Making use of prior art techniques, such highly nitrogen-containing substrates may be digested only under difficult conditions in biogas plants.
In the anaerobic fermentation, the used organic substances (such as, e.g., proteins, nucleic acids, fats and carbon hydrates) are incrementally degraded in the absence of oxygen by different anaerobic microorganisms into smaller compounds. Final products of the biogas process are methane (CH4), carbon dioxide (CO2) as well as not further degradable carbon and mineral compounds. The nitrogen bound in the biomass is released as ammonium (NH4+) and remains in the final fermentation product. Too high concentrations may inhibit the microbial process or even have toxic effects on the microorganisms. Independent of temperature and pH, ammonium is in equilibrium with ammonia (NH3), which is considered as cytotoxin and has an inhibitory effect already at small concentrations.
In order to enable for raw materials having high nitrogen concentrations (e.g., protein-rich by-products of slaughter houses, waste from the leather industry, residual substances of the bio fuel production, etc.) being processed in anaerobic fermentation processes, the development of new strategies is necessary.
For the microbiology in the fermenter it is important that certain amounts of nitrogen components are available. These are required for the growth of the biomass; the amount absorbed by the microorganisms, however, only constitutes a fraction of the nitrogen amount contained in the present substrate. In the course of the biogas process illustrated in FIG. 1, in the absence of dissolved oxygen, polymer compounds are hydrolyzed by different heterotrophic anaerobic microorganisms.
The nitrogen containing polymers, proteins and nucleic acids are degraded in the first step into amino acids, purines and pyrimidines. In the further course of the biological degradation process, nitrogen is released in the form of ammonium and remains—in contrast to the biologically available organic substances that are converted into biogas—in the fermentation liquid. In this way, the nitrogen degradation products are even concentrated. With increasing pH and increased temperatures in the process, the reaction equilibrium shifts from ammonium towards ammonia.
Ammonia has toxic effects on bacteria, as nearly all biological membranes are permeable to ammonia due to the small size of the NH3 molecule as well as the lipid solubility thereof.
Among the different microorganisms that are involved in the anaerobic digestion, the methanogenes are the ones being the least tolerant in regard to high concentrations of ammonium and, hence, most likely prone to inhibition and toxic effects. As a consequence of inhibition, this leads to small biogas yields and the accumulation of intermediate degradation products such as free volatile fatty acids. Apart from the reduced yields of renewable energy, the biological process hindrances further led to increased emissions of smell out of the final fermentation product due to insufficiently degraded substrates.
In order to solve the problem of nitrogen, there have so far been discussed in principle several approaches:                Exclusion of specific substrates having high nitrogen content,        Dilution of the nitrogen-rich substrate or        Separation of the nitrogen from the substrate.        
Though the exclusion and the dilution of specific substrates may be taken into consideration for individual plant operators, this way is, however, not successful as a solution to the present problem of the digestion of nitrogen-rich waste and by-products. Diluting the substrate, for example, with waste waters having low nitrogen concentrations, leads to an enormous demand of additional digestion space. At the same time, the efforts for the storage and the transport costs of the fermentation residues would be multiplied. The most successful approach, hence, is the discharge of the nitrogen component from the biogas process.
This procedure reduces the inhibition of the microorganisms by increased NH4+/NH3 concentrations and, hence, exerts the following positive influences on the biogas process:                Increased methane yields at stable fermenter volume        Higher degradation rates enable for higher space load        More stable biological process by preventing the accumulation of free volatile fatty acids (FFS), which also impede the efficiency of the process        Reduction of undesired emissions of smell due to complete degradation of the organic mass        
By way of separation of significant amounts of the ammonium nitrogen, there may be guaranteed in an existing biogas plant an improved and more stable operation, and the degradation performance and, consequently, the methane yield are remarkably increased. In this way, it is possible to increase the space load of the fermenters, which is why in the future the entire substrate spectrum as well as the maximum gas yield thereof may be used for the production of energy.
Resulting from this significant increase in the performance, there are provided correspondingly higher yields of CO2— neutral electricity and heat from the increased utilization of the combined thermal and power plants (BHKW). For the selected removal of ammonia from liquid media, there have already been developed a series of methods, in particular in the waste water technology. Some of these have been tested on a large-scale basis in part for a considerable period of time, thus being state of the art. For the relatively new sector of biogas, these technologies have been adapted and may be used in the field of fermentation residues processing. These technologies, however, are used only in exceptional cases due to the high efforts in terms of energy, operating materials and substrate pre-treatment.
In general, the procedures for removing nitrogen may be distinguished in biological and physical-chemical methods:
The nitrification/denitrification as a biological method represent an established technology of the biological waste water treatment. The removal of nitrogen, herein, is carried out in two partial steps:                1. Nitrification        The reaction of ammonium into nitrate under aerobic conditions is carried out by chemolithotrophic nitrifying bacteria. This oxidation is realized in two steps. Bacteria of the “nitroso group” are able to oxidize ammonia or ammonium up to the nitrite (NO2—) (ammonium oxidizing agents). The further oxidation into nitrate (NO3—) is performed by nitrobacteria types or other representatives of the “nitro group” (nitro oxidizing agents).        2. Denitrification        In the absence of oxygen, various aerobic waste water bacteria may oxidize organic compounds having nitrate instead of O2. In the denitrification process, there are developed gaseous final products from the nitrate, this is mainly molecular nitrogen (N2). As a by-product there may also be released nitrous oxide (laughing gas, N2O).        
As the first of the two reaction steps has to be performed in an aerobic atmosphere, this method that has often been tested in the waste water technology is unsuitable for the present case of the anaerobic biogas production, because in the aerobic step, the major part of the chemical energy would be used instead of being reacted into biogas.
The Anammox process (anaerobic ammonium oxidation) offers an alternative to the classic method of nitrification/denitrification. Therein, ammonium is reacted with nitrite under anaerobic conditions into molecular nitrogen. In spite of a plurality of descriptions in the literature, this process cannot be designated as prior art yet.
In regard to the biological methods, the advantage of the majority of physical-chemical methods is essentially that present ammonium nitrogen is not reacted into elementary nitrogen. The nitrogen thus is not released into the atmosphere but is rather available in different chemical compounds as a resource, depending on the method.
The removal of the dissolved nitrogen compounds from the fermentation medium may be carried out using so-called stripping methods. In general, stripping is the removal of volatile compounds from liquids using gas. By decreasing the partial pressure of the more volatile component, this transits from the dissolved state into the gaseous state in the gas phase and enriches in the liquid. The basic principle is represented as a unit having two main methodological steps:                Ammonia elimination from the substrate using stripping gas (air or vapour, respectively)        Regeneration of the stripping gas and transferral of the ammonium into a recyclable material flow        
In order to transfer the dissolved ammonia into the gaseous phase (desorption), there have been proven successful on a large-scale industrial basis methods of air and vapour stripping in filling body columns. The main characteristic thereof is that the liquid to be treated is brought into as intensive contact as possible with the gas flow. Therefore, a rather big mutual exchange area is of great importance. This is achieved by introducing filling bodies, at the surface of which the liquid phase will move along in the form of a film.
For the flawless operation of a stripping facility, the nearly complete removal of solids is a prerequisite, as otherwise this will lead to blocking/obstructing of the filling body coating. The level of the necessary removal of solids respectively depends on the type of filling bodies and the other facility structure.
With the aid of ion exchange methods, charged ingredients (ions, e.g., ammonium NH4+) may be adsorbtively bound and exchanged for ions having the same charge. There are predominantly used synthetic resins, so-called ion exchange resins. A rather comprehensive pre-treatment of the substrate, in particular the removal of solids, is a necessary prerequisite. Because of the structure of the ion exchange material, this is relatively sensitive in regard to the obstruction of the hollow spaces, for example, by organic colloids, whereby the performance is largely reduced. Thus the use is only profitable, according to the present state of the art, for post-treatment, e.g. following already performed nanofiltration or return osmosis in order to securely guarantee the given limit values, which is why it is unsuitable for the treatment of the fermentation medium that is aimed for.
A variant of the ammonium extraction recently examined is the use of extractive membranes in membrane contactors. Membrane contactors with pervaporation are novel apparatuses for performing extraction or stripping processes, respectively, in which the phase exchange takes place at a membrane surface. In conventional stripping columns the liquid and the gaseous phase are in direct contact, wherein the necessary intensive mixing is obtained by sprinkling with filling bodies. In membrane contactors, the two phases are present separated from each other by a membrane that is only permeable to gaseous ammonia.
In the connection with membrane reactors, however, there is existent a series of unanswered questions, such as, e.g., the long-term stability and proneness to fouling of the membranes. In total, these methods are still in an early phase of development, and presently it is still unclear whether realization and real implementation will be successful.